1. Technical Field
The present invention relates to a hard disk drive for magnetic storage of digital information and, more particularly, relates to a method of compensating for non-linear torque characteristics of a voice coil motor in order to reduce access time to a particular track of a magnetic disk and to reliably read and write data to and from the same particular track of magnetic disk.
2. Related Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Hard disk drives incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Such hard disk drives typically include a motor drive spindle and associated mechanism for receiving and rotating a disk at a predetermined constant speed, a servo control mechanism for radially positioning a read/write transducer head in response to track position commands, and electronic circuitry for operating the motor to provide data to and read data from the disk. Some disk drives are designed with only one read/write transducer head while others are designed with a plurality of transducer heads for enabling data to be written onto and read from a plurality of corresponding disks.
High performance disk drive systems commonly employ a moving coil actuator (e.g., voice coil motor) for positioning the transducer heads. Coarse positioning, which involves moving the transducer head from one data track to another during a seek mode, is accomplished by controlling the actuator and transducer head in a velocity feedback loop. After a coarse position move is executed, the feedback loop settles the transducer on the selected data track and continues following the data track during a track following mode for normal read/write operations. An important parameter in the track seek operations is the torque of the moving coil actuator. Ideally, the torque of a moving coil actuator is constant with position. The linearity of torque curve is important because it influences the access time of the disk drive. Any variation of the torque with angular position can lead to a longer seek operation and instability problems during track following operation. Contemporary voice coil motor designs with a uniform torque constant are disclosed, for example, in U.S. Pat. No. 4,908,816 for Voice Coil Motor With Linearized Force Constant issued to Champagne et al., U.S. Pat. No. 5,005,095 for Actuator For Linearizing Torque In A Disk Drive System issued to Wagner, U.S. Pat. No. 5,122,702 for Rotary Voice Coil Motor With A Flat Coil issued to Nakamoto, and U.S. Pat. No. 5,260,618 for Space Optimization Voice Coil Motor For Disk Drives issued to Dunfield et al.
Generally, servo control system calculates gain for moving a transducer head to a selected data track during a seek mode using a linearized torque constant. However, the real torque constants at the inner and outer tracks of the disk are comparatively smaller than those at the center track due to a rotation angle. This difference is caused from various external factors such as an unequal density of the magnetic field formed by permanent magnets, unstable bias-force caused by a spindle motor or a flexible printed circuit (FPC) cable, and friction of the actuator bearings. Consequently, track seek and track following operations become unstable at locations where the torque constant is much different from that of the center tracks. This instability causes increased vibration which decreases the performance and reliability of high performance disk drive.
Recent efforts to compensate for actuator torque non-linearity can be found, for example, in U.S. Pat. No. 5,182,685 for Stepper Motor Torque Variance Compensation Using Varied Applied Voltage issued to Krause et al., U.S. Pat. No. 5,305,160 for Compensating For Variations In Torque Capability Of Voice Coil Motors issued to Funches, U.S. Pat. No. 5,476,015 for Non-Linearity Compensation For Hard Disk Drives issued to Valent, and U.S. Pat. No. 5,616,869 issued to Actuator Torque Non-Linearity Compensation For Hard Disk Drive issued to Valent. In Valent '015 and '869, for example, the irregularity of the torque constant of a voice coil motor VCM is compensated using a torque constant multiplier table containing different values for different positions of a disk drive. While these efforts contain their own merits, I have observed that they are overly complex. Accordingly, a simpler and yet effective technique for compensating for non-linear torque characteristics of a voice coil motor VCM can still be contemplated.